Difference between a Sig 551 and 556?

[Balrog]

Can someone tell me the differnce? They seem cosmetically fairly similar.

 

[RCArms.com]

551 takes propritary Sig 551 mags only and has a paddle type mag release.

556 takes M16 mags and has a button release.

There are rear sight differences as well, but the magazine and lower is the major difference.

 

[Balrog]

Is the 556 a good rifle? It seems heavier than an AR. Is that because it is a piston mechanism?

 

[Ar180shooter]

The piston does add some weight. It's a long stroke piston design with a rotating bolt that was obviously heavily influenced by the AK47 action. It's robust and reliable and a good rifle IMO, since the major QC issues the earlier ones had have been resolved.

 

[RCArms.com]

I recently warmed up to my 556 Classic that I purchased some time ago. The Factory diopter sights are not as easy for me to use, but as soon as I mounted optics, the entire system really started to click for me.

Between the 556 and the 551, the 556 is the hands down winner for me because of M16 mag usage.

Don

 

[BrainOnSigs]

The 556 is essentially a 551-SP...the semi-auto version of the 551...the method of operation and mechanical function are identical...they differ in that the 556 has a lighter aluminum lower that can utilize NATO STANAG magagines...and the rear of the lower reciever is modified to be compatable with M4 type collapsing stocks...

...virtially all the parts are readily interchangable

...gas block assembly
...gas valve
...gas piston and spring
...gas tube

...the entire bolt carrier group

...bolt and all it's components
...carrier and all it's components
...firing pin and spring
...charging handle
...charging handle catch assembly

...the entire fire control group including springs
...selector assembly
...bolt catch assembly
...pressure point screw assembly
...take down / pivot pins
...bolt cover and hardware

...handguards
...pistol grip

...* the 556 / 551 barrel does not interchange...the rear is treaded differently...likewise the treading for a muzzle attachment differs...

...* the swiss lower will fit and function with the 556 upper...however...when attaching a 556 lower to a swiss upper a small amount of metal must be relieved from the lower portion of the trunion to accomadate the Nato STANAG magazines...

...* the swiss folding stock will not interchange with the collapsing stock...

...* the magazine catch assembly in the 551 differs from that in the 556...and will not interchange...

...* the magazines do not interchange...

 

[bushmaster1313]

Difference between a Sig 551 and 556?

5, give or take a few.

 

[stubbicatt]

Update to BrainonSig's posting. The present iteration of the 551 and the 556 Classic, Swat, "R", and Patrol models all use the folding swiss style buttstock. On my 556 Classic it locks up tight. I do not think that SIG is still making the collapsible AR15 style buttstock anymore on their rifles, though I understand that one may purchase an adapter and the necessary parts, even whilst retaining the side folding feature.

 

[Ro1911]

they used to be very different quality wise but now the 556 is a 551 upper on a upgraded 556 lower so the only difference is one(551) takes very expensive sig mags and the other(556) takes cheap AR15 mags.

 

[stubbicatt]

I must say that my 556 Classic is an impressive mechanism. I have not shot for groups so cannot comment on accuracy. It is however, minute of clay pigeon at 200 yards.